Storage devices are employed to store data that are accessed by computer systems. Examples of basic storage devices include volatile and non-volatile memory, floppy drives, hard disk drives, tape drives, optical drives, etc. A storage device may be locally attached to an input/output (I/O) channel of a computer. For example, a hard disk drive may be connected to a computer's disk controller. A storage device may also be accessible over a network. Examples of such a storage device include network attached storage (NAS) and storage area network (SAN) devices. A storage device may be a single stand-alone component or be comprised of a system of storage devices such as in the case of Redundant Array Of Inexpensive Disks (RAID) groups.
For mission-critical applications requiring high availability of stored data, various techniques for enhancing data reliability are typically employed. One such technique is to provide a “mirror” for each storage device. In a mirror arrangement, data are written to at least two storage devices. Thus, data may be read from either of the two storage devices so long as the two devices are operational and contain the same data. That is, either of the two storage devices may process read requests so long as the two devices are in synchronization.
When one of the storage devices fails, its mirror may be used to continue processing read and write requests.
In general, a data replication system can provide an up-to-the-minute duplicate copy or replica of changing data on a storage device. Write commands issued to a primary storage device are duplicated and issued to the data replication system, which records the written data in its own storage medium. Sophisticated data replication systems store not only a current duplicate copy of the primary device but also allow additional past-time images of the primary device to be accessed. This may be done through “journaling,” where the write commands themselves are archived, rather than simply a copy of the data.
Sometimes, however, communication to the data replication system is lost. This may be for a variety of reasons. For example, a physical connection with the device hosting the data replication system may be broken, or communication software may malfunction. When this happens, a data replication system will be out of synchronization with the primary storage device. Some reconciliation process is necessary to restore synchronization between the data replication system and the primary storage device.
Performing this reconciliation well is not a trivial task. One easy way to reconcile a primary storage device with its data replication system is simply to temporarily take the primary storage device out of service and copy the contents of the primary storage device to the data replication system. It is an undesirable technique, as it requires taking the primary storage out of service.
Another way to reconcile the two storage systems is to temporarily (while the data replication system is unavailable) store the duplicated write commands in an auxiliary journal, then “replay” the write commands for the data replication system when it comes back up.
As is known in the art, large host computer systems require large capacity data storage systems. These large computer systems generally include data processors which perform many operations on data introduced to the computer system through peripherals including the data storage system. The results of these operations are output to peripherals, including the storage system.
One type of data storage system is a magnetic disk storage system. Here a bank of disk drives and the computer system are coupled together through an interface. The interface includes “front end” directors (or controllers) and “back end” disk directors (or controllers, also known as rear end directors or disk directors). The interface operates the directors in such a way that they are transparent to the computer. That is, data is stored in, and retrieved from, the bank of disk drives in such a way that the computer system merely thinks it is operating with one large memory. One such system is described in U.S. Pat. No. 5,206,939, entitled “System and Method for Disk Mapping and Data Retrieval”, inventors Moshe Yanai, Natan Vishlitzky, Bruno Alterescu and Daniel Castel, issued Apr. 27, 1993, and assigned to the same assignee as the present invention.
As described in such U.S. Patent, the interface may also include, in addition to the front-end directors and disk directors, an addressable global cache memory. The global cache memory is a semiconductor memory connected to all of the front end directors and back end directors and is provided to rapidly store data from the computer system before storage in the disk drives, and, on the other hand, store data from the disk drives prior to being sent to the computer. The cache memory being a semiconductor memory, as distinguished from a magnetic memory as in the case of the disk drives, is much faster than the disk drives in reading and writing data.
In operation, when the host computer wishes to store end-user (i.e., host computer) data at an address, the host computer issues a write request to one of the front-end directors to perform a write command. One of the front-end directors replies to the request and asks the host computer for the data. After the request has passed to the requesting one of the front-end directors, the director determines the size of the end-user data and reserves space in the cache memory to store the request. The front-end director then produces control signals for such front-end director. The host computer then transfers the data to the front-end director. The front-end director then advises the host computer that the transfer is complete. The front-end director looks up in a Table, not shown, stored in the cache memory to determine which one of the rear-end directors is to handle this request. The Table maps the host computer address into an address in the bank of disk drives. The front-end director then puts a notification in a “mail box” (not shown and stored in the cache memory) for the rear-end directOr which is to handle the request, the amount of the data and the disk address for the data. Other rear-end directors poll the cache memory when they are idle to check their “mail boxes”. If the polled “mail box” indicates a transfer is to be made, the rear-end director processes the request, addresses the disk drive in the bank, reads the data from the cache memory and writes it into the addresses of a disk drive in the bank. When end-user data previously stored in the bank of disk drives is to be read from the disk drive and returned to the host computer, the interface system operates in a reciprocal manner. The internal operation of the interface (e.g. “mail-box polling”, event flags, data structures, device tables, queues, etc.) is controlled by interface state data (sometimes referred to as metadata) which passes between the directors through the cache memory. Further, end-user data is transferred through the interface as a series of multi-word transfers, or bursts. Each word transfer in a multi-word transfer is here, for example, 64 bits. Here, an end-user data transfer is made up of, for example, 32 bursts. Each interface state data word is a single word having, for example, 64 bits.
As is also known in the art, a disk drive contains at least one magnetic disk which rotates relative to a read/write head and which stores data nonvolatilely. Data to be stored on a magnetic disk is generally divided into a plurality of equal length data sectors. A typical data sector, for example, may contain 512 bytes of data. A disk drive is capable of performing a write operation and a read operation. During a write operation, the disk drive receives data from a host computer (e.g., here, a back end director) along with instructions to store the data to a specific location, or set of locations, on the magnetic disk. The disk drive then moves the read/write head to that location, or set of locations, and writes the received data. During a read operation, the disk drive receives instructions from a host computer to access data stored at a specific location, or set of locations, and to transfer that data to the host computer. The disk drive then moves the read/write head to that location, or set of locations, senses the data stored there, and transfers that data to the host.
The host computer, which for some purposes may include the storage system itself, may not address the disk drives of the storage system directly, but rather access to data may be provided to one or more host computers from what the host computers view as a plurality of logical devices or logical volumes (LVs), also referred to as LUNs. The LUNs may or may not correspond to the actual disk drives. For example, one or more LUNs may reside on a single physical disk drive. In another example, a LUN may use storage space from multiple physical disk drives. An LV or LUN (logical unit number) may be used to refer to the foregoing logically defined devices or volumes.
In the industry there have become defined several levels of RAID systems. The first level, RAID-0, combines two or more drives to create a larger virtual disk. In a dual drive RAID-0 system one disk contains the low numbered sectors or blocks and the other disk contains the high numbered sectors or blocks, forming one complete storage space. RAID-0 systems generally interleave the sectors of the virtual disk across the component drives, thereby improving the bandwidth of the combined virtual disk. Interleaving the data in that fashion is referred to as striping. RAID-0 systems provide no redundancy of data, so if a drive fails or data becomes corrupted, no recovery is possible short of backups made prior to the failure.
RAID-1 systems include one or more disks that provide redundancy of the virtual disk. One disk is required to contain the data of the virtual disk, as if it were the only disk of the array. One or more additional disks contain the same data as the first disk, providing a “mirror” of the data of the virtual disk. A RAID-1 system will contain at least two disks, the virtual disk being the size of the smallest of the component disks. A disadvantage of RAID-1 systems is that a write operation must be performed for each mirror disk, reducing the bandwidth of the overall array. In a dual drive RAID-1 system, the first disk and the second disk contain the same sectors or blocks, each disk holding exactly the same data.
RAID-2 systems provide for error correction through hamming codes. The component drives each contain a particular bit of a word, or an error correction bit of that word. RAID-2 systems automatically and transparently detect and correct single-bit defects, or single drive failures, while the array is running. Although RAID-2 systems improve the reliability of the array over other RAID types, they are less popular than some other systems due to the expense of the additional drives, and redundant onboard hardware error correction.
RAID-4 systems are similar to RAID-0 systems, in that data is striped over multiple drives. For example, the storage spaces of two disks are added together in interleaved fashion, while a third disk contains the parity of the first two disks. RAID-4 systems are unique in that they include an additional disk containing parity. For each byte of data at the same position on the striped drives, parity is computed over the bytes of all the drives and stored to the parity disk. The XOR operation is used to compute parity, providing a fast and symmetric operation that can regenerate the data of a single drive, given that the data of the remaining drives remains intact. RAID-3 systems are essentially RAID-4 systems with the data striped at byte boundaries, and for that reason RAID-3 systems are generally slower than RAID-4 systems in most applications. RAID-4 and RAID-3 systems therefore are useful to provide virtual disks with redundancy, and additionally to provide large virtual drives, both with only one additional disk drive for the parity information. They have the disadvantage that the data throughput is limited by the throughput of the drive containing the parity information, which must be accessed for every read and write operation to the array.
RAID-5 systems are similar to RAID-4 systems, with the difference that the parity information is striped over all the disks with the data. For example, first, second, and third disks may each contain data and parity in interleaved fashion. Distributing the parity data generally increases the throughput of the array as compared to a RAID-4 system. RAID-5 systems may continue to operate though one of the disks has failed. RAID-6 systems are like RAID-5 systems, except that dual parity is kept to provide for normal operation if up to the failure of two drives.
Combinations of RAID systems are also possible. For example, a four disk RAID 1+0 system provides a concatenated file system that is also redundant. The first and second disks are mirrored, as are the third and fourth disks. The combination of the mirrored sets forms a storage space that is twice the size of one individual drive, assuming that all four are of equal size. Many other combinations of RAID systems are possible.
In at least some cases, when a LUN is configured so that its data is written across multiple disk drives in the striping technique, the LUN is operating in RAID-0 mode. Alternatively, if the LUN's parity information is stored on one disk drive and its data is striped across multiple other disk drives, the LUN is operating in RAID-3 mode. If both data and parity information are striped across multiple disk drives, the LUN is operating in RAID-5 mode.
Advances in semiconductor technology have lead to an increase in the use of a semiconductor solid state drive (also known as a solid state disk or SSD) which uses a flash memory as a storage device, in areas such as computer systems. Thus, in at least some cases there seems to be a trend towards the use of an SSD as a storage device instead of a magnetic disk. In spite of having features such as, for example, a relatively small storage capacity and a relatively high price, the SSD has some other features that can make it more attractive as a storage device than the conventional magnetic disk in at least some cases.
Features that can make SSDs preferable as storage devices are, for example, a fast access rate, high throughput, a high integration density, and stability against an external impact. SSDs can move much larger amounts of data and process far more I/O requests, per time period, than conventional magnetic disks. This allows users to complete data transactions much more quickly.
Furthermore, advances in manufacturing technologies for SSDs may reduce the production costs of SSDs and also increase the storage capacities of SSDs. These developments may provide further incentive to use SSDs in place of magnetic disks in at least some cases.
Solid state disk systems may also comprise communication controllers, such as Fibre Channel (FC) controllers, Ethernet mechanisms, ATA or serial ATA interfaces, or SCSI controllers for managing data communication with external computing devices.